The present invention relates to power generation from low-temperature heat, either from waste heat, solar-thermal sources or geothermal sources, using hydro-osmotic processes for generating electrical energy at a thermal efficiency greater than 30%, well in excess of heat engine cycles for such low-temperature heat.
Captured and reused waste heat is an emission-free substitute for costly purchased fuels or electricity. Numerous technologies are available for converting waste heat to power. Nonetheless, anywhere around 513 quadrillion Btu/yr of waste heat energy remains unrecovered or unconverted to power. The United States industrial sector accounts for approximately one third of all energy used in the United States, consuming approximately 32 quadrillion Btu (1015 Btu) of energy annually, and emitting about 1,680 million metric tons of carbon dioxide associated with this energy use. During these manufacturing processes, as much as 20 to 50% of the energy consumed is ultimately lost via waste heat contained in streams of hot exhaust gases and liquids, as well as through heat conduction, convection, and radiation from hot equipment surfaces and from heated product streams.
The efficiency of generating power from waste heat recovery is heavily dependent on the temperature of the waste heat source. In general, economically feasible power generation from waste heat has been limited primarily to medium- to high-temperature waste heat sources (i.e. >500° F.). Emerging technologies, such as organic Rankine cycles, are beginning to lower this limit, though they are hampered by low power conversion efficiency, and further advances in alternative power cycles are needed for economic feasibility of power generation at lower temperatures.
The largest amount of waste heat is in the low-temperature group, defined as waste heat in the temperature region of 150° C. to 275° C., although higher temperatures may still be considered low-temperature in the context of waste heat energy conversion. All of the various technologies currently being investigated for such a temperature regime have relatively low efficiency and high capital costs, when used for low-temperature waste heat sources for power generation.
Heat sources at different temperatures have varying theoretical efficiency limits for power generation. Maximum efficiency of a heat engine to run a power generation system at a given temperature is based on the Carnot efficiency. The Carnot efficiency represents the maximum possible efficiency of an engine at a given temperature. The Carnot efficiency increases for higher temperatures and drops dramatically for lower temperatures, with heat engine efficiency becoming very low for temperature differences of 150-200° C. (η≈15-20%). In addition, a heat exchanger's or a recuperator's surface area increases disproportionately with lower temperature waste heat sources, limiting their economics.
Some applicable technologies for power generation from low-temperature waste heat are Steam Rankine Cycles (SRC), wherein power generation from waste heat involves using the heat to generate steam in a waste heat boiler, which then drives a steam turbine. In the Steam Rankine Cycle, the working fluid, typically water, is first pumped to elevated pressure before entering a heat recovery boiler. The pressurized water is vaporized by the hot exhaust and then expanded to lower temperature and pressure in a turbine, generating mechanical power that can drive an electric generator. The low-pressure steam is then exhausted to a condenser at vacuum conditions, where heat is removed by condensing the vapor back into a liquid. The condensate from the condenser is then returned to the pump and the cycle continues. Organic Rankine Cycles (ORC), involve other working fluids with better efficiencies at lower heat source temperatures used in ORC heat engines. ORCs use an organic working fluid that has a lower boiling point, higher vapor pressure, higher molecular mass, and higher mass flow compared to water. Together, these features enable higher turbine efficiencies than in an SRC. ORC systems can be utilized for waste heat sources as low as 300° F., whereas steam systems are limited to heat sources greater than 500° F. ORCs have commonly been used to generate power in geothermal power plants, and more recently, in pipeline compressor heat recovery applications. The Kalina cycle involves a mixture of water and ammonia as the working fluid, which allows for a more efficient energy extraction from the heat source. The Kalina cycle has an operating temperature range that can accept waste heat at temperatures of 200° F. to 1,000° F. and is 15 to 25 percent more efficient than ORCs at the same temperature level. Kalina cycle systems are becoming increasingly popular overseas in geothermal power plants, where the hot fluid is very often below 300° F. Even liquid carbon dioxide has been proposed as a working fluid.
SRCs are the most familiar to industry and are generally economically preferable where the source heat temperature exceeds 800° F. For lower temperatures, ORC or Kalina cycle systems are used. They can be applied at temperatures lower than for steam turbines, and they are more efficient in moderate temperature ranges. Kalina systems have the highest theoretical efficiencies. Their complexity makes them generally suitable for large power systems of several megawatts or greater. ORC systems can be economically sized in small, sub-megawatt packages, and they are also well suited for using air-cooled condensers, making them appropriate for applications such as pipeline compressor stations that do not have access to water.
In addition to Rankine cycle systems, there are a number of other advanced technologies in the research and development stage that can generate electricity directly from heat, and that could in the future provide additional options for power generation from waste heat sources. These technologies include thermoelectric, piezoelectric, thermionic, and thermo-photovoltaic (thermo-PV) devices that use solid state systems that require no moving parts and sit directly in the waste stream. Several of these have undergone prototype testing in automotive applications and are under development for industrial heat recovery. Utilizing liquid streams below 200° F. and gas streams below 500° F. typically remains economically impractical with today's technologies, however. Conversion to electricity is less efficient with all these technologies, compared to traditional electric generators, and project costs currently run high for a variety of reasons, including the cost of the equipment and the cost of integrating the waste heat recovery system with the waste heat source.
The total cost to install waste heat to power (WHP) systems include the costs associated with the waste heat recovery equipment (boiler or evaporator), the power generation equipment (steam, ORC, or Kalina cycle), power conditioning and interconnection equipment. It would also include the soft costs associated with designing, permitting and constructing the system. The installed costs of Rankine cycle power systems (steam, ORC or Kalina) are fairly similar, differing more as a function of project size and the complexity of site integration than type of system.
All of the various technologies detailed above have relatively low efficiency and high capital costs when used for low-temperature waste heat sources for power generation. Typically, the Kalina cycle (water-ammonia mixtures) has an efficiency of around 12-15%, while thermo-electric generators have an efficiency of 5-7%. Piezo-electric generators currently have an efficiency of around 1%, and much developmental work needs to be done before they are cost-effective. Thermo-ionic generators and thermo-photovoltaic systems are still being investigated at the laboratory scale for low-temperature WHP applications.
Thus, there is a significant industrial and environmental need for a power generation system for conversion of low-temperature waste heat (≈150-275° C.) to electrical energy, not limited by the ideal Carnot cycle efficiency.
Forward osmosis (FO) is a technology currently being explored for desalination of seawater. Unlike reverse osmosis (RO) processes, which employ high pressures ranging from 400-1100 psi to drive fresh water through a membrane, forward osmosis uses the natural osmotic pressures of salt or polymer solutions, called ‘draw solutions’, to effect fresh water separation. A draw solution having a significantly higher osmotic pressure than the saline feed-water, flows along the permeate side of the FO membrane, and water naturally transports itself across the membrane by osmosis. Osmotic driving forces in FO can be significantly greater than hydraulic driving forces in RO, leading to higher water flux rates and recoveries. Thus, it is a low-pressure system, allowing design with lighter, compact, less expensive materials. These factors translate in considerable savings, both in capital and operational costs.
Joint research by Yale University and Oasys Inc, under an Office of Naval Research grant, compared forward osmosis to reverse osmosis processes, and found superior performance and flux rates. Based on these studies, Oasys developed a forward osmosis process using ammonium bicarbonate aqueous solutions as the draw down liquids. Other FO processes have been proposed, using either magnesium chloride draw solutions, polymeric draw solutions based on polyethylene glycols, volatile solutes like dimethyl amines, sulfur dioxide or aliphatic alcohols, or bivalent/precipitable salts like aluminum sulfate/calcium hydroxide (Modern Water, UK). Glucose or sucrose have been used as solutes for the draw solution, which can then be ingested after suitable dilution (Hydration Technologies International Inc). Polymeric draw solutions have also been developed based on polyethylene glycols (PEGs) and polypropylene glycols (PPGs).
Solutions of magnesium chloride, ammonium chloride, calcium chloride in water, and polymers like PEG/PPG solutions in water generate very high osmotic pressures, in the range of 300-400 atm, based on their concentration. The ionic salts mentioned above, as well as sodium and potassium bicarbonates, also do not decompose or scale at the temperatures contemplated herein, while the water in the salt solution can be substantially boiled off by the application of low-temperature waste heat, thus regenerating the concentrated salt solutions needed for hydro-osmotic power generation. The preferred draw solute for this application would be the ionic chlorides of magnesium or calcium, due to their very high osmotic potentials at a concentration of 2.5M to 3.0M, as well as the minimized scaling of these salts at steam temperatures. The use of these salts also enables the temperature in the boiler/heat exchanger to be higher, called the Top Brine Temperature (TBT) to around 125-150° C., which increases the efficiency of the boiler. However, the main drawback in the use of these concentrated ionic solutions is the need to boil off and recover the water of dilution, since the latent heat of vaporization of water is around 970 Btu/lb of water to be removed, a substantial energy penalty.
Similarly, polymeric draw solutions also generate very high osmotic potentials, and are also not volatile, with very high boiling points 230° C.), suitable for power generation from low-temperature waste heat. A polyethylene glycol (PEG) solution generates very high osmotic pressures for its solutions in water, depending on its concentration. Thus, a 95% solution in water of PEG 400 at 20° C. has a calculated osmotic pressure of 658 atm; for PEG 600, it is 977 atm; for PEG 2000, it is 2540 atm.
Polyethylene glycols (PEGs), polymers of ethylene glycol (EG), have been used in industry to produce very high osmotic pressures, in the order of tens of atmospheres. In comparison, seawater (3.5% NaCl) has an osmotic pressure of only 28 atms at 25° C. PEGs are hypotonic by nature, and absorb water exceedingly well. The hydrogen bonding between water molecules and the electron-rich ether oxygen in the EO (ethylene oxide) monomer enables almost 2.5-3.0 molecules of water to be coordinated with each EO monomer, leading to high osmotic pressures. Thus, the greater the number of EO monomers in the PEG molecule, the greater the osmotic pressure exhibited. One issue with longer chain-length PEGs is higher viscosity and higher melting points, as the chain length increases. PEG 200 (EO=4), PEG 300 (EO=6-7) and PEG 400 (EO=9) are all liquid at room temperatures, whereas PEG 600 (EO=12-13) is a waxy solid at room temperature, as are the higher molecular weight PEGs. Thus, a practical limit in the PEG chain length prevents use of increasingly longer chain-length PEGs for water absorption.
These polymers, by suitable chemical modification (like propoxylation, butoxylation or addition of fatty acids or fatty bases to their chains) can also be rendered hydrophobic at higher temperatures, called “cloud point” or critical point temperatures. If an hydrophobic entity, like propanediols or butanediols or fatty acids/bases, is attached to the PEG molecule, the hydrophobic-lipophilic balance (HLB) of the copolymer can be suitably shifted, such that phase separation can occur at certain temperatures, usually termed cloud-point or critical temperatures, as mentioned in the paragraph above. The draw solute copolymers consist of various numbers and orders of diols, which impart the required solution properties. Osmotic pressure, cloud point temperature, molecular weight and molecular structure can be adjusted by adding or subtracting the various monomer units. Within the constraints of osmotic pressure and cloud point temperature, the chemistry of the draw solute polymers can be selected to control the molecular weight (preferably greater than 600) and/or physical structure of the polymer (preferably branched) resulting in high (>90% and preferably >99%) rejection of the draw solute through filtration. Further, the chemistry of the draw solute polymers can be selected to incorporate larger molecules to minimize back diffusion of the solute through the forward osmosis membrane.
Such “cloud point” polymers, also called thermo-sensitive polymeric solutions, have been considered as suitable osmotic draw solutes. These polymers have a tendency for phase separation from their water solutions at a critical temperature, and thus can be suitably separated from the permeated water of the FO process. Both lower and upper critical temperatures have been exhibited, depending on the configuration of the polymer molecule. At the lower critical temperature, the polymer separates into a hydrophobic layer from the water, and thus, can be re-concentrated by nano-filtration or other techniques for recycling as a concentrated draw solute for the next cycle of FO. Some polymers can re-dissolve in water above the upper critical temperature.
While the PEGs used in these copolymers are linear in structure, and increase in melting point and viscosity as the chain-length increases, there are other forms of PEGs available, with different geometries, that are termed branched or multi-armed PEGs. Branched PEGs have 3-10 PEG chains emanating from a central core group. Star PEGS have 10 to 100 PEG chains emanating from a central core group, while comb PEGs have multiple PEG chains grafted onto a polymer backbone. Such branched PEGs allow more EO groups in the polymer, while remaining in the liquid state and having lower melting points and viscosity than comparable linear PEGs with the same number of EO monomers. Thus, the use of such PEG geometries can enable higher water absorption, while retaining the practicality of using higher number of EO monomers for water molecule interaction by hydrogen bonding. An additional property of these branched PEG polymers, as described in co-pending U.S. patent application Ser. No. 15/271,175, filed Sep. 20, 2016, and Ser. No. 15/272,406, filed Sep. 21, 2016, the entire contents of each of which are incorporated herein in their entirety by reference, is also the ability to phase-separate from water by suitable amine-termination of the branched ends of these polymers and subsequent absorption of carbon dioxide.
The preferred engineered polymers, for the practical application of embodiments of this invention for power generation from low-temperature waste heat would be polymers with a high osmotic potential, preferably around 400-600 atms, but low critical temperatures for phase separation from their water mixtures. The hydrophilic-lipophilic balance (HLB) of polymers defines the water solubility, osmotic potential and the cloud point temperature of these engineered polymers. The higher the HLB ratio, the higher the osmotic potential, but also the higher the cloud point temperature. While, traditionally, the use of similar polymers in desalination and saline waste-water treatment systems, limits the HLB ratio to around 10-13, to keep the cloud point temperature lower than 60° C., for this particular application of hydro-osmotic power generation, an HLB ratio of around 14-17 is preferred, but an associated phase separation temperature of below 70-85° C. Such properties of suitably engineered polymers enable high flux rates against fresh water across the FO modules, while efficiently phase-separating at temperatures associated with low-temperature waste heat (≈150-275° C.), without inordinately large heat transfer surfaces. Some such polymers would be block or random branched co-polymers of ethoxylate-propoxylates like sorbitol ethoxylate-propoxylates, sorbitan ethoxylate-propoxylates, glycerol ethoxylate-propoxylates, trimethylolpropane ethoxylate-propoxylates, pentaerithritol ethoxylate-propoxylates, glucose and sucrose ethoxylate-propoxylates, other poly-hydric polymers, and similar branched derivatives of these ethoxylate-propoxylate co-polymers.
Modification of these polymer derivatives by amine-termination enables them to undergo phase-separation from water, or inverse solubility in water, by absorption of CO2, as described in co-pending U.S. patent application Ser. No. 15/271,175, filed Sep. 20, 2016, and Ser. No. 15/272,406, filed Sep. 21, 2016, the entire contents of each of which are incorporated herein in their entirety by reference. Use of such polymers, with their high osmotic pressures, and their property of inverse solubility with water by CO2 absorption, can be used for hydro-osmotic power generation.
A great quantity of energy can be potentially obtained when waters of different salinities are mixed together. The harnessing of this energy for conversion into hydro-osmotic power can be accomplished by means of a technology called Pressure Retarded Forward Osmosis (PRFO). This technique uses a semi-permeable membrane to separate a less concentrated solution, or solvent, (for example, fresh water) from a more concentrated and pressurized solution (for example an osmotic draw agent), allowing the water to pass to the concentrated solution side. The difference in osmotic potential between two solutions, separated by a semi-permeable membrane, yields a pressure differential, which is similar to the effect of gravity in creating potential energy (static head) for conversion to hydroelectric energy. Normal hydropower plants use the static head of water in dams to yield energy when the water is allowed to run through turbine generators. Similarly, osmotic pressure differentials can also be used to drive hydro-turbine generators to create energy. The additional fluid volume due to the permeation of water increases the pressure on the permeate side, which is depressurized in a hydro-turbine to produce power—thus the term ‘hydro-osmotic power’.
The use of the above-mentioned engineered polymers are useful for hydro-osmotic power generation. It is estimated that the thermal efficiency of “ionic salt” based osmotic power is less than 5%. Even the use of common cloud-point polymers has a substantial energy penalty, since the entire polymer-water mixture has to be heated up to the cloud point temperature for inducing phase separation. It is estimated that the thermal efficiency of “cloud-point polymer” based osmotic power is less than 7%.